Debugging a Minidump



Table of Contents




Use minidump_stackwalk to show a stack trace


TODO(mkrebs): ...





Use gdb to show a backtrace

Generate core file

Convert a minidump to a core file.
sh -c '~/checkouts/cros/cros6/chroot/usr/bin/minidump-2-core -v upload_file_minidump-7adc2ee0079cb374.dmp > minidump.core 2>minidump.core.out'
For a minidump from a 32-bit executable, use minidump-2-core.32 instead.

For an ARM minidump, you have to work a little bit harder to get a core file.  The easiest way is probably to do the conversion using qemu within your chroot.
# Setup ARM libraries for running qemu
cp -a /usr/armv7a-cros-linux-gnueabi /tmp/qemu-arm
cp -a /usr/lib/gcc/armv7a-cros-linux-gnueabi/4.7.x-google/* /tmp/qemu-arm/lib/
# Convert minidump to a core file in /tmp/
qemu-arm -L /tmp/qemu-arm/ /build/daisy/usr/bin/minidump-2-core -v ~/test/upload_file_minidump-de1f11232d825812.dmp > /tmp/minidump.core 2>/tmp/minidump.core.out
Reference: http://crosbug.com/34938

Calculate address for symbols

First, setup the necessary files for the debugger.  You need both the original executables and/or libraries whose symbols you wish to calculate, and you will need their corresponding debug information.  Googlers: See Setup files for debugger for how to get these for official images.

Calculate the base address of the crashing executable's .text section.
# Determine base address where the crashing executable was mapped
set t1=`grep -w GUID minidump.core.out | grep '"/opt/google/chrome/chrome"' | sed -e 's/-.*//'`
# Determine offset of .text within the executable (run objdump with sudo if setuid program like Xorg)
set t2=`objdump -h /usr/local/google/home/mkrebs/tmp/test/sigabrt/2913.45.0/image/rootfs/opt/google/chrome/chrome | \grep '\.text'|awk '{print $4}'`
# Print out address of .text in memory
perl -e 'die unless $ARGV[0] && $ARGV[1]; printf("%#x\n", hex($ARGV[0]) + hex($ARGV[1]))' $t1 $t2

You may also want to do this for shared libraries that the executable loaded.
# Determine base address where the library was mapped
set t1=`grep -w GUID minidump.core.out | grep '"/lib/libc-2.15.so"' | sed -e 's/-.*//'`
# Determine offset of .text within the executable (run objdump with sudo if setuid program like Xorg)
set t2=`objdump -h /usr/local/google/home/mkrebs/tmp/test/issue35349/2913.84.5/image/rootfs/lib/libc-2.15.so | \grep '\.text'|awk '{print $4}'`
# Print out address of .text in memory
perl -e 'die unless $ARGV[0] && $ARGV[1]; printf("%#x\n", hex($ARGV[0]) + hex($ARGV[1]))' $t1 $t2

Googlers: Alternatively, you can try my generate_gdb_command_file script to automatically generate a gdb command file for adding all the symbol files.  For the "--top" command-line option, specify the path to where your image is mounted.  Then, pass the path to the file to which you redirected minidump-2-core's stderr.
/home/mkrebs/bin/scripts/generate_gdb_command_file --top 2913.84.5/image/rootfs/ ~/checkouts/cros/cros6/chroot/tmp/minidump.core.out > minidump.gdb

Start debugger

Run gdb on the core file.
gdb --core minidump.core
armv7a-cros-linux-gnueabi-gdb --core minidump.core

Map executable's symbol file to base address of .text section.
(gdb) add-symbol-file <path to top-level "debug" directory>/<path to .debug file> <address from calculation>

Googlers: Alternatively, if you tried my generate_gdb_command_file script, use the generated gdb command file to add all the symbol files.
gdb --core minidump.core --command minidump.gdb

Example

grep -w GUID minidump.core.txt | grep '"/opt/google/chrome/chrome"' | sed -e 's/-.*//'
0x72CC9000
objdump -h /usr/local/google/home/mkrebs/tmp/test/sigabrt/2913.45.0/image/rootfs/opt/google/chrome/chrome | \grep '\.text'|awk '{print $4}'
002a73e0
perl -e 'printf("%#x\n", hex($ARGV[0]) + hex($ARGV[1]))' 0x72CC9000 002a73e0
0x72f703e0

gdb --core=minidump.core
(gdb) add-symbol-file /usr/local/google/home/mkrebs/tmp/test/sigabrt/2913.45.0/debug/opt/google/chrome/chrome.debug 0x72f703e0



If backtrace in gdb did not help

Sometimes, the gdb backtrace command (Use gdb to show a backtrace) doesn't show a stack trace any better than that of minidump_stackwalk (Use minidump_stackwalk to show a stack trace).  If you think there's more to it than what those two are showing you, try this method to naively dump all the known symbol addresses seen on the stack.  You'll see some false positives, but you may just find the name of a function that seems like a plausible place to look.

First, start from the above step of using gdb to show a backtrace.  We can reuse the gdb command file that was generated for it.  Googlers: If you didn't use my generate_gdb_command_file script, you can manually create the gdb command file containing any "add-symbol-file" commands you ran.

Second, determine the base address of the stack.  The easiest way is to just look at the stack address in the minidump.  This command will set the $sp_addr variable to the stack address of the first thread in the minidump:
set sp_addr=`~/checkouts/cros/cros6/chroot/usr/bin/minidump_dump upload_file_minidump-62893577d8a73070.dmp | perl -ne 's/^  stack.start_of_memory_range = // && print && exit'`

If you want to be smarter, you can run minidump_stackwalk on the minidump (using the appropriate Breakpad symbols) and skip past any stack frames you can assume are accurate.  You would do this by looking for the stack pointer of the last consecutive call frame found by "call frame info".  For example, for issue http://crosbug.com/35531 the stack pointer value would be 0x7e8b3440 given the following top-most (i.e. higher address) frames:
  7  Xorg!OsSigHandler [osinit.c : 146 + 0x11]
      r4 = 0x76fc9f7c    r5 = 0x7e8b3440    r6 = 0x76bfb094    r7 = 0x00000000
      r8 = 0x00000000    r9 = 0x00000020   r10 = 0x00000004    fp = 0x00000008
      sp = 0x7e8b3430    pc = 0x76f90949
     Found by: call frame info
  8  libc-2.15.so + 0x25e6e
      r4 = 0x774ec898    r5 = 0x76e9aeb8    r6 = 0x76bfb094    r7 = 0x00000000
      r8 = 0x00000000    r9 = 0x00000020   r10 = 0x00000004    fp = 0x00000008
      sp = 0x7e8b3440    pc = 0x76b40e70
     Found by: call frame info
  9  libc-2.15.so!new_do_write [fileops.c : 537 + 0x11]
      sp = 0x7e8b345c    pc = 0x76b6abe5
     Found by: stack scanning

Finally, run the following gdb command to dump 1024 words of the stack (i.e. potential addresses).  In the output, for any value that corresponds to a known symbol, gdb will annotate it with the symbol's name in angle brackets.  Knowing that, we can simply pipe the output to something (in this case, perl) to pull out any annotation it finds in the stack dump.
gdb --core minidump.core --batch --command minidump.gdb --eval-command "x/1024aw $sp_addr" |& perl -ne '$re=qr{(<((?:[^<>]++|(?-2))*+)>)}; print "$2\n" while (s/0x[0-9a-f]+ $re//)'





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